1. Field of the Invention
This invention relates to luminance signal/color signal separating circuits and noise reduction circuits, and more particularly, to luminance signal/color signal separating circuits and noise reduction circuits using comb filters in magnetic recording reproducing devices such as video cassette recorders (hereinafter referred to as VCR).
2. Description of the Background Art
A VCR has been proposed using a 1H delay (DL) circuit comprising one CCD (Charge Coupled Device), operating as a comb filter during recording for separating an input video signal into a luminance signal (Y) and a color signal (C) (Y/C separation), and operating as a noise reduction circuit during reproduction for removing the crosstalk components in the reproduced color signal as well as removing the small noise components in the reproduced luminance signal, such as the one disclosed in Japanese Patent Laying-Open No. 59-107691 for example.
Y/C separation by a comb filter using a 1H delay circuit is well known, as disclosed in U.S. Pat. No. 3,764,739. A color TV signal (hereinafter referred to as video signal) of the NTSC system has the spectrum of luminance signal components and the spectrum of color signal components alternately interleaved as shown schematically in the frequency spectrum diagram of FIG. 1 to avoid beat disturbance on the screen. Generally, when there is no difference between adjacent upper and lower horizontal lines, that is, when there is line correlation, luminance signal components are concentrated in the vicinity of the spectrum of n times (n is integer) the horizontal frequency f.sub.H (solid lines Y.sub.1), while color signal components are concentrated in the vicinity of the spectrum of (n+1/2) f.sub.H (broken lines C). That is, Y signal components and C signal components are mutually frequency interleaved by (1/2) f.sub.H. On the contrary, when there is difference between adjacent upper and lower horizontal lines, that is, when there is no line correlation, non-correlation components Y.sub.2 and Y.sub.3 of the Y signal are generated respectively in the low frequency region and the high frequency region of the complete band of signal Y in a frequency interleaved relation with the frequency multiplied output Y.sub.1 of the horizontal frequency (FIG. 1).
For example, in a VCR recording mode, a filter having a comb type band-pass characteristic periodically repeated in accordance with the repetition of the spectrum shown in FIG. 1 is required to separately extract luminance signal components and color signal components from the input video signal having such a frequency spectrum.
A filter having such a comb band-pass characteristic is implemented by adding or subtracting an input video signal (Y+C) and a signal delayed by 1H by employing a 1H delay circuit, by means of a circuit shown in FIG. 3 for example. In the event of the above two signals being added, a passing characteristic with the peak at nf.sub.H and the valley at (n+1/2) f.sub.H is realized as shown in FIG. 2(a) to pass the luminance signal components Y.sub.1. On the other hand, in the event of the above two signals being subtracted, a passing characteristic with the valley at nf.sub.H and the peak at (n+1/2) f.sub.H is realized as shown in FIG. 2 (b) to pass the color signal components C.
During a recording mode of a VCR, a Y/C separating circuit using the above mentioned comb filter will extract luminance signal components in accordance with the comb type band-pass characteristic shown in the above FIG. 2 (a) with respect to the complete band of the luminance signal. Therefore, the non-correlation components Y.sub.2 and Y.sub.3 in the low and high frequency regions of Y in (1/2) f.sub.H frequency interleaved relation with respect to Y.sub.1 which is the frequency multiplied output of the horizontal period f.sub.H are not extracted in the low and high frequency regions of the above mentioned complete band and will be excluded from the luminance signal components. This eliminates the luminance signal variation between the upper and lower lines, that is, in the vertical direction, resulting in a problem of degradation in vertical resolution of the picture on the screen.
A 1H delay circuit used as a Y/C separating filter in a recording mode will be used as a noise reduction circuit during a VCR reproduction mode for removing the mixture of low frequency converted color signal components from adjacent tracks on a tape, that is, crosstalk, in the reproduction system of the color signal. In addition, this 1H delay circuit will be used as a noise reduction circuit for removing small noise components in a luminance signal in the reproduction system of the luminance signal. However, since such a noise reduction circuit will suppress not only the noise components in the luminance signal, but also the small signal components as noise, there was a problem that the small signal components will be decreased in the reproduced luminance signal.
Furthermore, generally in the reproduction mode, it is regarded that conventional VCRs will compensate for the missing reproduction signal, that is, will perform dropout compensation using the 1H delayed signal of the adjacent horizontal period reproduced signal when the reproduced signal is missing, i.e., when dropout occurs in a horizontal period. However, the output of the 1H delay circuit constituting the above mentioned comb filter has the phase of the color subcarrier 180.degree. shifted with respect to the missing reproduced signal to be compensated. Therefore, the output of the 1H delay circuit may not be used for dropout compensation as it is.